1. Field of the Invention
The present invention relates generally to a data transmitting/receiving apparatus for a base station (BS) and a mobile station (MS), and in particular, to an apparatus and method for transmitting/receiving packet data between a BS and an MS.
2. Description of the Related Art
In general, a mobile communication system transmits/receives data in a predetermined frequency band to ensure mobility for users. The driving force behind the recent rapid development of mobile technology is the growing demands for data services such as wireless Internet browsing. Considering the unexpectedly soaring number of users in mobile communication systems, there is a need to explore a method of efficiently providing limited resources to multiple users in designing a mobile communication system.
A voice service-focusing mobile communication system requires minimization of time delay, whereas a data communication supporting system requires minimization of an error rate. In view of the characteristic of data communication, the existing circuit-switched protocol gives rise to an improved packet data protocol. Developmental work and standardization are under way to enable services using the packet data protocol to be available in existing mobile communication systems such as GSM (Global System for Mobile Communication), CDMA (Code Division Multiple Access), and DAMPS (Digital Advanced Mobile Phone System).
Concerning the European mobile communication system, GSM, it provides circuit-switched data services and connectivity to an external data communication network. The circuit-switched data services are used for packet-switched data communication as well as for circuit-switched data communication. GPRS (General Packet Radio Service) has been introduced into the GSM to render the packet-switched data communication efficient. The GPRS enables IP (Internet Protocol) communication or virtual circuit-switched communication. The GPRS supports a connectionless protocol like IP as well as a connection-oriented protocol like X.25. One of the advantages of the packet-switched data protocol is sharing one transmission resource among a plurality of users. In GSM, a time slot of the radio frequency carrier can be shared among a plurality of users during transmission/reception. The shared resources for uplink and downlink transmission are managed by a network, that is, a base station of a mobile communication system.
The major advantages of implementation of a packet data protocol in a mobile communication system are high rate data transmission and efficient use of frequency bands. The GPRS is, briefly saying, a “multi-slot operation” which allows one user to occupy one or more transmission resources.
FIG. 1 illustrates a GPRS network configuration according to the GPRS standards.
Referring to FIG. 1, packets input to an external X.25 network 122 and an external IP network 124 are transferred to a GPRS network through GGSNs (Gateway GPRS Support Nodes) 120. The packets are transmitted to SGSNs (Serving GPRS Support Nodes) 116 though a backbone network 118 by path selection in the GGSNs 120. Each SGSN 116 supports a service for the area where a GPRS terminal is located. The SGSN 116 transmits a GPRS packet destined for the terminal to a BSS (Base Station Subsystem) 110 for dedicated transmission. A GPRS register in an HLR (Home Location Register) 114 stores all GPRS service subscriber data. The subscriber data are exchanged between the SGSN 116 and an MSC (Mobile Switching Center) 112 to check service-related items such as limited roaming.
FIG. 2 illustrates the structure of transmission packet data. Referring to FIG. 2, packet transmission will be described below.
A packet from a GPRS network is mapped to at least one LLC (Logical Link Control) frame. The LLC frame has an information field, a frame header (FH), and a frame check sequence (FCS). This LLC frame is mapped to a plurality of RLC (Radio Link Control) data blocks. Each RLC data block includes a block header (BH), an information field, and a block check sequence (BCS). Here, “block” is the smallest transmission unit for a packet in the air interface. That is, an RLC data block is mapped to a radio block of a physical layer. L3H is the header of Layer 3 message.
Three medium access modes are supported in the RLC/MAC layer of GPRS: dynamic, extended dynamic, and fixed. The MS, in dynamic allocation, monitors its assigned USF (Uplink State Flag) on each PDCH (Packet Data Physical Channel) and transmits one or four radio blocks on the PDCH. The extended dynamic allocation is a simple extension of the dynamic one adapted to deliver large volume data. Within this mode, a USF value indicates assigned block periods on several PDCHs. In the case of the fixed allocation, a certain amount of assigned block periods are fixed. Handling with this mode does not involve monitoring of a USF value for a half duplex mode. A medium access mode available to the MS depends on MAC_MODE in a PACKET DOWNLINK ASSIGNMENT message received from the BS.
In the description of the present invention, a link and a channel directed from the BS to the MS are termed a “downlink” and a “downlink channel”, and a link and a channel directed from the MS to the BS are termed an “uplink” and an “uplink channel”.
The RLC BH has a USF for medium access on the uplink. FIG. 3 illustrates the structure of RLC/MAC data. The structure of RLC/MAC data block is defined and explained in GSM/GPRS specification. A radio block (also called and RLC/MAC block) consists of one MAC header, one control message contents or one data message contents. MAC header contains;    USF(Uplink state flag, 3 bits) This is used to identify users for Uplink transmission, or to characterize a PRACH.    Payload Type(2 bits) This identifies the type of block that follows (RLC data block or RLC/MAC control block).    S/P(Polling control, 3 bits) One Supplementary/Polling(S/P) bit to poll the mobile station (so that it sends an acknowledgement message) and two RRBP bits to tell the mobile station where to send the acknowledgement messages.The USF of a packet data channel is used for multiplexing uplink radio blocks from a plurality of mobile users. The USF occupies three information bits and thus identifies eight (23) USF states. Uplink traffic is multiplexed according to the USF value. The USF is included in each radio block at the initial downlink transmission. Since the USF is set in all downlink radio, dynamic allocation is used. All MSs sharing a particular transmission resource monitor USFs on a downlink channel to check whether the uplink transmission resource is available. If its assigned USF indicates to an MS that uplink transmission is available, the MS transmits data in the next uplink radio block:
FIG. 4 is a timing diagram showing uplink transmission according to the USF of a downlink channel. Referring to FIG. 4, for USF=R1, MS1 is authorized to use four uplink bursts. Similarly, for USF=R2, MS2 is authorized to use four uplink bursts. For USF=F, an MS transmits an uplink PRACH (Packet Random Access Channel) to initiate uplink transmission. In the above manner, uplink transmission is carried out.
For uplink packet transmission, the MS transitions from a packet idle mode to a packet transfer mode and requests resource assignment to the BS by a Packet Channel Request message. The BS then determines whether resources are available by checking the state of radio resources. If the radio resources are available, the BS establishes a TBF(Temporary Block Flow) mode and transmits a Packet Uplink Assignment message to allow the MS to use one or more PDCHs. At the same time, the BS sets a USF value in a PDCH for identification of the resource-assigned MS. The MS sets a timer to TBF Starting Time set in the Packet Uplink Assignment message and uses a PDCH when the timer expires. This is the uplink contention resolution of GPRS.
The Packet Uplink Assignment message has a USF_GRANULARITY field. If USF_GRANULARITY is 0, one uplink radio block per USF is transmitted. If USF_GRANULARITY is 1, four uplink radio blocks per USF are transmitted. Then, the BS checks the radio block(s) received from the MS and transmits a Packet Uplink Ack/Nack message to the MS.
FIGS. 5 and 6 illustrate establishment of the uplink according to the standard specification GSM/GPRS 05.02 when USF_GRANULARITY is 0 and 1, respectively. UN is Unused downlink slot. The case with USF_GRANULARITY=1 exhibits better system performance than the case with USF_GRANULARITY=0.
It may occur that when the MS is about to complete packet data transmission, transmission data is generated in the BS. Such cases are quite common due to the bidirectionality of packet data communication.
In the above-described conventional technology, the MS transitions from the packet transfer mode to the packet idle mode after packet transmission. The MS then acquires system-related information from a PBCCH(Packet Broadcast Control Channel) and checks whether a Packet Paging Request message has been received by monitoring a PCCCH(Packet Common Control Channel). If the Packet Paging Request includes a paging message for the MS, the MS sets a dedicated mode by transmitting a Packet Paging Response message as defined in GSM 04.60 to the BS and receives data from the BS, which will be described in connection with FIG. 7.
FIG. 7 is a diagram illustrating a signal flow for establishing the uplink, transmitting uplink data, and then establishing the downlink according to the GSM standards.
Upon generation of data to be transmitted to the BS, the MS is released from the packet idle mode and transmits the Packet Channel Request message to the BS. If the BS determines that a packet channel is available by checking the radio links, it transmits the Packet Uplink Assignment message to the MS. As described referring to FIG. 5 or FIG. 6, the BS acknowledges packet uplink assignment by a USF. Then, the MS transmits packet data until there are no remaining packet data to be transmitted as indicated by the USF.
After the packet transmission is completed, if data to be transmitted to the MS is generated in the BS, the BS checks a PBCCH and pages the MS by the Packet Paging Request message. Upon receipt of the paging signal in the packet idle mode, the MS transmits the PACKET PAGING RESPONSE signal to the BS. Thus, a dedicated mode is established between the BS and the MS and downlink data transmission is carried out.
If transmission data for the MS is generated at the end of packet transmission from the MS, the BS has no way to notify the MS that it has transmission data for the MS. Therefore, the dedicated mode is not set until the MS transitions from the packet transfer mode to the packet idle mode and then receives the Packet Paging Request message paging the MS from the BS.
In other words, after releasing the established radio resource (RR) connection between the BS and the MS, the BS sets a new TBF mode, pages the MS and transmits data to the MS an inefficient use of resources and time.